Rolling shutter.
Some cameras don't address all the pixels at once, reading them out sequentially, usually top to bottom. This is most clearly visible on things like in webcams, cellphones, stills cameras that also shoot video, and the like, but also some higher end and broadcast cameras to a lesser extent.
This is a particularly lovely diagrammatic demonstration of why it happens:
https://www.youtube.com/watch?v=17PSgsRlO9Q
Many broadcast and other higher-end cameras don't do it quite as badly as that, but things like cellphones often do, especially at high shutter speeds (where fast moving objects aren't blurred, which can mask the effect). It can cause other problems with brief or fast-moving phenomena, such as "flash banding" caused by muzzle flashes from weapons fire or xenon strobes that are visible for only part of the frame, as here, with photographic flashguns.
Objects moving horizontally can end up looking like this:
P